1. Field of the Invention
The present invention relates to a semiconductor laser device.
2. Description of the Related Art
A conventional semiconductor laser device 800 will be described with reference to FIGS. 8A through 8C.
FIGS. 8A and 8B are cross-sectional views illustrating a front end face and a rear end face of the conventional semiconductor laser device 800, respectively. FIG. 8C is a diagram illustrating a top plan view of a cross-section of the conventional semiconductor laser device 800, as taken along the X-Y line indicated in FIGS. 8A and 8B.
As shown in FIGS. 8A and 8B, the semiconductor laser device 800 includes: a semiconductor substrate 1 made of an n-type InP material and having a mesa structure; a light confinement layer 2 made of an n-type InGaAsP material (band-gap wavelength: about 1.05 xcexcm) and having a thickness of about 600 nm; an active layer 3 having a multiple quantum-well structure: a light confinement layer 4 made of a p-type InGaAsP material and having a thickness of about 600 nm; and a cladding layer 5 made of a p-type InP material and having a thickness of about 400 nm. The layers 2 through 5 are provided in this order on the mesa region of the semiconductor substrate 1. The active layer 3 includes seven InGaAsP well layers (not shown) each having a thickness of about 6 nm and a compressive distortion of 1.0% or less, and seven InGaAsP (band-gap wavelength: about 1.05 xcexcm) barrier layers (not shown) each having a thickness of about 10 nm and no compressive distortion, such that the InGaAsP well layers and the InGaAsP barrier layers are alternately layered on one another.
The semiconductor laser device 800 also includes: a first buried layer 6 made of a p-type InP material whose carrier density is 7.0xc3x971017 cmxe2x88x923, a second buried layer 7 made of an n-type InP whose carrier density is 2.0xc3x971018 cmxe2x88x923, a third buried layer 8 made of a p-type InP whose carrier density is 7.0xc3x971017 cmxe2x88x923, and a contact layer 9 made of a p-type InGaAsP (band-gap wavelength: about 1.3 xcexcm). The layers 6 through 9 are provided in this order in the vicinity of the active layer 3. In order to achieve acceptable reverse I-V characteristics in the semiconductor laser device 800, the buried layers 6 through 8 are formed so as to have pnp-buried type different densities from one another.
In order to reduce the parasitic capacity in the buried layers 6 through 8 so as to improve the frequency response characteristics of the semiconductor laser device 800, grooves are provided around the stripe structure by using an etching technique. The grooves extend into the first buried layer 6.
On the contact layer 9, a SiO2 film 10 is formed having a thickness of about 0.3 xcexcm with an aperture therein. A metal multilayer film 11 including three layers (i.e., an Au layer, a Zn layer, and an Au layer) is formed in the aperture, and a p-type electrode 12 is formed on the metal multilayer film 11. An n-type electrode 13 is provided on the backside of the semiconductor substrate 1.
Referring to FIG. 8C, a top plan view of a cross-section of the active layer 3 of the semiconductor laser device 800 is shown. The active layer 3 has a width of about 0.6 xcexcm in a region within about 25 xcexcm from the front end face, while it has a width of about 1.6 xcexcm in a region within about 25 xcexcm from the rear end face. The distance between the front end face and the rear end face is about 400 xcexcm, and the cross-section of the active layer 3 has a stripe structure. The width of the active layer 3 having this stripe structure continuously decreases from the rear end face toward the front end face. Thus, the stripe width of the active layer 3 at the front end face is narrower than that at the rear end face. This is a structure of a semiconductor laser device for implementing narrow output angle characteristics and low operation current characteristics at a high temperature (Y. Inaba et al., IEEE JSTQE, vol. 3, 1399-1404, 1997). With this structure, the effect of confining light within the active layer 3 continuously decreases from the rear end face toward the front end face. Therefore, a large amount of light leaks out of the active layer 3 into the first buried layers 6 in the region adjacent to the front end face. Moreover, light also leaks out of the active layer 3 into the third buried layer 8 because the light confinement layer 4 and the cladding layer 5 are thin.
FIG. 9 is a graph illustrating the relationship between an operation ambient temperature and an output angle of the conventional semiconductor laser device 800. As seen from FIG. 9, when the operation ambient temperature changes from about xe2x88x9240xc2x0 C. to about 85xc2x0 C., the output angle changes from about 14.0xc2x0 to about 10.2xc2x0 (i.e., by about 3.8xc2x0). Therefore, in a case where light output from the semiconductor laser device 800 is coupled to an optical fiber, for example, such temperature changes affect the coupling efficiency between the light from the semiconductor laser device 800 and the optical fiber, thereby affecting the intensity of the light propagated through the optical fiber. This will adversely affect the transmission characteristics of the optical fiber.
It is understood that the output angle changes because there exists about a 0.025 difference in the refractive index between the first buried layer 6 and the second buried layer 7, and between the third buried layer 8 and the second buried layer 7. This problem is especially prominent in a semiconductor laser device having an active layer with a small width (i.e., the stripe width is shorter in comparison with the wavelength) since a large amount of light leaks out of the active layer 3. The amount of change in the intensity of the light propagated through the optical fiber should satisfy the practical standards in optical communications (i.e., 1 dB or less for a temperature change of about xe2x88x9240xc2x0 C. to about 85xc2x0 C.). Otherwise, the transmission characteristics of the optical fiber would be very poor, and thus, the semiconductor laser device 800 might not be usable. The conventional semiconductor laser device 800 does not satisfy the above-mentioned practical standards since the amount of change in the intensity of the light propagated through the optical fiber is 2 dB.
As one type of conventional semiconductor laser device in which the active layer has a constant width with respect to the longitudinal cross-section of resonator, a semiconductor laser device is known which includes a single buried layer made of InP-based material doped with Fe (H. Taniwatari et. al., IEEE, JLT, vol. 15, 534 to 537). InP-based material doped with Fe, however, has a problem in that Fe diffuses into InP during a high-temperature operation. As a result, a current leak occurs and therefore it is difficult to achieve long-term reliability in such a semiconductor laser device.
In a conventional semiconductor laser device with pnp-type buried layers, such as the semiconductor laser device 800, the buried layers have different densities from one another. Therefore, the output angle of the beam changes depending on the amount of current applied to the laser and the temperature condition. As a result, it is difficult to achieve stable output angle characteristics.
In one aspect of the present invention, a semiconductor laser device includes: a semiconductor substrate; an active layer having a stripe structure formed on the semiconductor substrate; and a buried layer formed on the semiconductor substrate and in a vicinity of the active layer, the buried layer including Fe and Ti.
In one embodiment of the present invention, the buried layer includes InP.
In another embodiment of the present invention, the active layer has a width which is smaller than a diameter of a light emitting spot formed adjacent to the active layer.
In still another embodiment of the present invention, the width of the active layer is smaller at a front end face than at a rear end face.
In still another embodiment of the present invention, a diffraction lattice is formed on the semiconductor substrate.
In still another embodiment of the present invention, the semiconductor laser device further including an optical fiber provided at the front end face, light which has been output from the front end face being coupled into the optical fiber.
In one aspect of the present invention, a semiconductor laser device includes: a semiconductor substrate; an active layer having a stripe structure formed on the semiconductor substrate; and a buried layer formed on the semiconductor substrate and in a vicinity of the active layer, the buried layer including Rh.
In one embodiment of the present invention, the buried layer includes InP.
In another embodiment of the present invention, the active layer has a width which is smaller than a diameter of a light emitting spot formed adjacent to the active layer.
In still another embodiment of the present invention, the width of the active layer is smaller at a front end face than at a rear end face.
In still another embodiment of the present invention, a diffraction lattice is formed on the semiconductor substrate.
In still another embodiment of the present invention, the semiconductor laser device further including an optical fiber provided at the front end face, light which has been output from the front end face being coupled into the optical fiber.
In one aspect of the present invention, a semiconductor laser device includes: a semiconductor substrate; an active layer having a stripe structure formed on the semiconductor substrate; and a buried layer formed on the semiconductor substrate and in a vicinity of the active layer, the buried layer having a uniform refractive index, wherein a width of the active layer is smaller at a front end face than at a rear end face.
In one embodiment of the present invention, a diffraction lattice is formed on the semiconductor substrate.
In another embodiment of the present invention, an absorption layer is formed on the diffraction lattice.
In still another embodiment of the present invention, a semiconductor laser device further including an optical fiber provided at the front end face, light which has been output from the front end face being coupled into the optical fiber.
Thus, the invention described herein makes possible the advantages of providing a semiconductor laser device in which the output angle is less dependent on temperature and thus long-term reliability is improved.
A semiconductor laser device according to the present invention includes a semiconductor substrate, an active layer having a stripe structure formed on the semiconductor substrate, and a buried layer made of an InP-based material doped with Fe and Ti, and formed on the semiconductor substrate and in the vicinity of the active layer.
According to the present invention, excessive donors and acceptors compensate each other by the actions of Fe and Ti contained in the buried layer of the semiconductor laser device. Therefore, a thermally stabilizing effect is provided, whereby the current leak due to thermal diffusion of Fe is reduced.
If thermally stable Rh is used instead of Fe, doping of Ti is not necessary.
The semiconductor laser device according to the present invention includes a buried layer having a uniform density, and therefore the refractive index of the buried layer is uniform. Therefore, the change in the output angle caused by a temperature change is controlled so as to be low. The effect of the present invention becomes especially remarkable in a semiconductor laser device in which the width of the active layer is smaller than the diameter of a light emitting spot formed in the vicinity of the active layer.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.